Rockwell Automation 1799-DASCII DeviceNet Serial Interface - Manual (Page 9)

DeviceNet Serial Interface Module 9
Publication 1799-IN012B-EN-P - July 2008
Change-of-State I/O
The change-of-state (COS) connection is the same as the cyclic connection except
that the COS connection also initiates a transfer on the receipt of the delimiter or an
overflow. The COS connection is mutually exclusive with the cyclic connection,
but can coexist with the polled connection. The COS connection operation is
useful in conserving bandwidth and provides the master with the most current data
as fast, or faster, than a poll connection. The COS connection automatically turns
on the COS mechanism when the connection is created.
Set Up the Connection Timer
The 1799-DASCII module sets the connection timer to the expected packet rate
(EPR), sometimes referred to as scan rate, value for the cyclic and polled
connection. It also uses this value in the connections to calculate the time the
device waits before signaling a timeout. If you are using a scanner or scanning
software, the EPR must be configured. The scanner configures the EPR in the
1799-DASCII module at the beginning of communication. Refer to the scanner
manual for how to configure the EPR.
Serial Data Strings and the Scanner's I/O Rate
Serial gateways typically use more bandwidth on the DeviceNet bus than other
types of I/O devices. If long data strings are implemented and/or multiple devices
are on the DeviceNet network, make sure the scan rate is set accordingly.
IMPORTANT
The EPR can be set manually by performing a set (Service 10hex) on the
connection class (Class 5hex) attribute 9. The polled connection uses
instance 2. The COS and cyclic connections use instance 4. This must be done
after allocating the connection.